1. Field of the Invention
The present invention relates to a process for producing pyromellitic acid by oxidizing 2,4,5-trimethylbenzaldehyde. Pyromellitic acid is useful as a raw material of special plasticizers, polyamides, imides, and the like.
2. Description of the Prior Art
The process for producing aromatic carboxylic acids by oxidizing aromatic hydrocarbons having an alkyl group or a partially oxidized substituent is stated in U.S. Pat. Nos. 2,276,774 and 2,245,528, wherein is disclosed a process using a lower aliphatic carboxylic acid such as acetic acid as a solvent in the presence of a heavy metal and a bromine compound.
This process is disadvantageous in that it uses a lower aliphatic carboxylic acid as solvent and hence combustion of the solvent inevitably takes place and that the process must involve a step for recovering the solvent.
In view of above, Japanese Patent Publication No. 13,921/64 disclosed a process which comprises oxidizing an aromatic compound in the presence of bromine ion in an aqueous solvent, and Japanese Patent Publication No. 2,222/83 disclosed a process which comprises oxidizing 2,4-dimethylbenzaldehyde or 2,4,5-trimethylbenzaldehyde in an aqueous solvent in the presence of a catalyst consisting of a bromine compound and a metal selected from manganese and cerium.
Although the process which comprises oxidizing 2,4,5-trimethylbenzaldehyde in a water solvent is advantageous in that solvent is not lost by combustion and a recovery of solvent is unnecessary, it is disadvantageous in that the oxidation rate is low and a high yield is not readily achievable.
That is, due to the low reactivity, intermediate products such as methyltrimellitic acid and the like remain in the resulting reaction mixture which makes the purification of pyromellitic acid difficult. Further, when the oxygen concentration in waste gas is enhanced by increasing the flow rate of air or when the reaction temperature is elevated or the catalyst concentration is altered, the amount of combustion increases to increase the formation of carbon dioxide which results in a decrease in the product yield.